Interventional medical systems, devices, and methods of use

ABSTRACT

A relatively compact implantable medical device includes a fixation member formed by a plurality of fingers mounted around a perimeter of a distal end of a housing of the device; each finger is elastically deformable from a relaxed condition to an extended condition, to accommodate delivery of the device to a target implant site, and from the relaxed condition to a compressed condition, to accommodate wedging of the fingers between opposing tissue surfaces at the target implant site, wherein the compressed fingers hold a cardiac pacing electrode of the device in intimate tissue contact for the delivery of pacing stimulation to the site. Each fixation finger is preferably configured to prevent penetration thereof within the tissue when the fingers are compressed and wedged between the opposing tissue surfaces. The pacing electrode may be mounted on a pacing extension, which extends distally from the distal end of the device housing.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to U.S. Provisional PatentApplication having the Ser. No. 62/041,954, which was filed on Aug. 26,2014, and is hereby incorporated by reference in its entirety. Thepresent application is related to the commonly assigned U.S. patentapplication Ser. No. 14/518,211, now U.S. Pat. No. 9,675,798, which isfiled concurrently herewith and entitled INTERVENTIONAL MEDICAL SYSTEMS,DEVICES, AND COMPONENTS THEREOF, and which is incorporated by referencein its entirety.

FIELD OF THE DISCLOSURE

The present disclosure pertains to interventional medical systems, andmore particularly to relatively compact implantable medical devicesthereof and associated methods.

BACKGROUND

The traditional implantable cardiac pacemaker includes a pulse generatordevice to which one or more flexible elongate lead wires are coupled.The device is typically implanted in a subcutaneous pocket, remote fromthe heart, and each of the one or more lead wires extends therefrom to acorresponding electrode, coupled thereto and positioned at a pacingsite, either endocardial or epicardial. Mechanical complications and/orMRI compatibility issues, which are sometimes associated with elongatelead wires and well known to those skilled in the art, have motivatedthe development of implantable cardiac pacing devices that are whollycontained within a relatively compact package, the entirety of which isconfigured for implant in close proximity to the pacing site. FIG. 1 isa schematic diagram that shows potential cardiac implant sites for sucha device, for example, within an appendage 102 of a right atrium RA,within a coronary vein CV (via a coronary sinus ostium CSOS), or inproximity to an apex 103 of a right ventricle RV. An implantingphysician may employ a standard guiding catheter (not shown) to delivera relatively compact medical device to any one of the three exemplarysites, for example, according to methods known in the art ofinterventional cardiology, by maneuvering the catheter, with the deviceloaded therein, up through the inferior vena cava IVC and into the rightatrium RA. However, a co-pending and commonly assigned U.S. patentapplication having the Ser. No. 14/039,937 discloses a moresophisticated delivery tool, which the operator may employ, in lieu ofthe standard guiding catheter, to deliver and to fix the device at thedesired implant site.

SUMMARY

A relatively compact implantable medical device, according toembodiments of interventional medical systems disclosed herein, includesa fixation member formed by a plurality of fingers mounted around aperimeter of a distal end of a housing of the device; each finger iselastically deformable from a relaxed condition to an extendedcondition, to accommodate delivery of the device to a target implantsite, and from the relaxed condition to a compressed condition, toaccommodate wedging of the fingers between opposing tissue surfaces atthe target implant site, wherein the compressed fingers hold a cardiacpacing electrode of the device in intimate tissue contact for thedelivery of pacing stimulation from the implanted device to the site.According to some methods, after an operator navigates a delivery tool,which has the device loaded within a distal-most portion of a deploymenttube thereof, through a venous system of the patient, to locate thedistal-most portion of the tool in proximity to a target implant site,the operator retracts the deployment tube with respect to the loadeddevice to expose the fixation fingers out through a distal opening ofthe lumen of the deployment tube, so that the fixation fingers arereleased from the extended condition to the relaxed condition; and thenthe operator advances the delivery tool toward the target site to wedgethe exposed fixation fingers between opposing tissue surfaces at thetarget implant site, thereby compressing the fixation fingers, so thatthe compressed fingers, by a spring force thereof, hold the pacingelectrode of the device in intimate tissue contact.

Each finger of the device fixation member, according to someembodiments, includes a first segment, which extends from a fixed end ofthe corresponding finger, and a second segment that extends from thefirst segment to a free end of the corresponding finger, wherein eachsecond segment extends in a distal direction, when the first segment ofeach finger is in the extended condition, and extends in a proximaldirection, outward from the device housing, when the first segment ofeach finger is in the relaxed condition. Furthermore, the second segmentof each fixation finger is preferably configured to prevent penetrationthereof within the tissue when the fingers are compressed and wedgedbetween opposing tissue surfaces.

According to some preferred embodiments, the pacing electrode of thedevice is mounted on a pacing extension of the device, wherein thepacing extension extends distally from the distal end of the devicehousing. When the device is loaded in the distal-most portion of theaforementioned delivery tool, an entirety of the pacing extension may becontained within the distal-most portion, along with a remainder of thedevice and the distal end of the inner member of the tool, which abuts aproximal end of the device housing, and each fixation finger, in theextended condition, extends in a distal direction alongside the pacingextension. According to some embodiments and methods, after navigatingthe delivery tool to locate the distal-most portion of the tool inproximity to the target implant site, the operator can retract thedeployment tube only enough to expose the pacing electrode of the pacingextension without exposing the fixation fingers, and then advance thedelivery tool toward the target site until the exposed electrode comesinto contact with tissue at the site, so that the operator can evaluatepacing performance at the site. If pacing performance is acceptable atthe site, the operator can pull the tool and loaded device back awayfrom the site, retract the deployment tube even further, with respect tothe loaded device, to expose the fixation fingers, and then advance thetool again toward the site to wedge the exposed fixation fingers betweenopposing tissue surfaces at the target implant site, thereby compressingthe fixation fingers, so that the compressed fingers hold the pacingelectrode in intimate tissue contact.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings are illustrative of particular embodiments of thepresent invention and therefore do not limit the scope of the invention.The drawings are not to scale (unless so stated) and are intended foruse in conjunction with the explanations in the following detaileddescription. Embodiments will hereinafter be described in conjunctionwith the appended drawings wherein like numerals denote like elements,and:

FIG. 1 is a schematic diagram showing potential implant sites forembodiments of the present invention;

FIG. 2A is a perspective view of an implantable medical device,according to some embodiments;

FIGS. 2B-C are elevation and end views of a fixation member componentwhich may be employed by the device of FIG. 2, according to someembodiments;

FIG. 3 is a plan view of an interventional medical system with a partialcut-away section, according to some embodiments;

FIGS. 4A-C are schematics outlining some methods of the presentinvention;

FIG. 5A is a perspective view of an implantable medical device,according to some additional embodiments;

FIG. 5B is a cross-section view through a portion of the device of FIG.5A, according to an exemplary construction of some embodiments; and

FIGS. 6A-C are schematics according to some alternate methods of thepresent invention; and

FIG. 7 is a perspective view of an implantable medical device, accordingto some alternate embodiments.

DETAILED DESCRIPTION

The following detailed description is exemplary in nature and is notintended to limit the scope, applicability, or configuration of theinvention in any way. Rather, the following description providespractical examples, and those skilled in the art will recognize thatsome of the examples may have suitable alternatives.

FIG. 2A is a perspective view of an implantable medical device 200,according to some embodiments. FIG. 2A illustrates device 200 includinga hermetically sealed housing 205, preferably formed from abiocompatible and biostable metal such as titanium, which contains apulse generator (e.g., a power source and an electronic controller—notshown), a fixation member, which is formed by a plurality of fixationfingers 230 spaced apart from one another around a perimeter of a distalend 201 of housing 205, and an electrode 261, which is located at thedistal end 201 of housing 205 being coupled to the controller of device200 by a hermetic feedthrough assembly (not shown) constructed accordingto those known to those skilled in the art of implantable medicaldevices. Housing 205 may be overlaid with an insulative layer, forexample, medical grade polyurethane, parylene, or silicone, and FIG. 2Afurther illustrates another electrode 262 of device 200, which may beformed by removing a portion of the insulative layer to expose themetallic surface of housing 205. According to the illustratedembodiment, electrode 262 may function in conjunction with electrode 261for bipolar pacing and sensing, when fixation fingers 230 hold electrode261 in intimate tissue contact at a target implant site, for example,within right atrial appendage 102 or within right ventricle RV inproximity to apex 103 (FIG. 1). Fixation fingers 230 function to holddevice 200 at the implant site by being wedged between opposing tissuesurfaces at the site.

FIGS. 2B-C are elevation and end views of an exemplary fixation membercomponent 23 which may be employed by device 200, according to someembodiments. FIGS. 2B-C illustrate fixation member component 23including eight fixation fingers 203 integrally formed with one anotherand a base ring 239, such that a thickness t of base ring 239 isapproximately the same as that of each finger 230. According to anexemplary embodiment, fixation member component 23 is cut from Nitinoltubing, according to methods known in the art, and thickness t may be0.005 inch +/−0.001 inch, wherein base ring 239 may have an innerdiameter id of approximately 0.20 inch and an outer diameter od ofapproximately 0.21 inch. A height x of base ring 239 may beapproximately equal to a width w of each finger, for example,approximately 0.024 inch. After cutting the aforementioned Nitinoltubing, fingers 230 are shaped by bending and holding fingers 230 in theillustrated curvature while heat treating component 23 according tomethods known to those skilled in the art. FIG. 2B illustrates (viacross-section through section line B-B of FIG. 2C) each fixation finger230 including a first segment 231 and a second segment 232, wherein eachfirst segment 231 extends from a fixed end 235 of the correspondingfinger 230 to the corresponding second segment 232, and each secondsegment 232 extends from the corresponding first segment 231 to a freeend 236 of the corresponding finger 230. FIGS. 2A-B further illustrateseach first segment 231, in a relaxed condition, extending in an arc,distally and outwardly from fixed end 235, and second segment 232extending from first segment 231 in a proximal direction and outwardfrom device housing 205. With further reference to FIG. 2C fixationfingers 230 are spaced equally apart from one another such that an angleψ defined between each adjacent pair is approximately 45 degrees.Component 23 may be mounted to distal end 201 of device housing 205, forexample, in a manner similar to that described for a fixation component102 in co-pending and commonly assigned United States Patent Application2012/0172690, which description is hereby incorporated by reference.

According to the illustrated embodiment, first segment 231 of eachfixation finger 230 is elastically deformable between the relaxedcondition and an extended condition, per arrow E of FIG. 2B, and betweenthe relaxed condition and a compressed condition, per arrow C of FIG.2B. The extended condition is described below in conjunction with FIGS.3, 4A, and 6A-B; and the compressed condition is described below inconjunction with FIGS. 4C and 6C. With further reference to FIG. 2B, theangle enclosed by the arc of first segment 231 of each finger 230 isshown being at least 90 degrees, with second segment 232 extending awayfrom first segment 231 at an angle θ. According to an exemplaryembodiment, a radius r of the arc of each first segment 231 isapproximately 0.067 inch, and angle θ is approximately 26 degrees. FIG.2B further illustrates each second segment 232 extending in a proximaldirection from first segment 231 over a distance just slightly greaterthan a distance z, wherein distance z may be approximately 0.095 inchmeasured from a proximal edge of base ring 239 to a tangent lineextending from an intersection of first and second segments 231, 232.Although not shown in FIG. 2A, according to some preferred embodiment,electrode 261 may be mounted on a relatively short extension formed indistal end 201 of housing 205 such that electrode 261 is spaced distalto radius r of each finger 230, for example, as shown in FIGS. 3 and 4B,wherein the distance from the apex of radius r to the distally spacedelectrode 261 may be approximately 2 mm. Furthermore, it should be notedthat if the exemplary dimensions of component 23, presented inconjunction with FIGS. 2B-C, are scaled down, for example, in proportionto a smaller overall implantable device volume, they will still fallwithin the scope of embodiments of the present invention.

FIG. 3 is a plan view of an interventional medical system with a partialcut-away section, according to some embodiments, wherein the systemincludes a delivery tool 300, in which device 200 is loaded, fordeploying device 200 to a target implant site. FIG. 3 illustrates tool300 including a handle 310, an elongate inner member 320, and an outerassembly, which is formed by an elongate deployment tube 330 and anouter, stabilizing sheath 370 that is secured to handle 310 andsurrounds a proximal portion of deployment tube 330 in proximity tohandle 310. According to the illustrated embodiment, elongate innermember 320 extends within a lumen 335 of deployment tube 330, and aproximal end of deployment tube 330 is coupled to a control member 312of handle 310 such that an entirety of deployment tube 330 is movablewith respect to the inner member 320, via control member 312. FIG. 3further illustrates inner member 320 including a distal end 322, whichis located within a distal-most portion 332 of deployment tube 330, andwhich is configured to engage implantable medical device 200 by abuttingproximal end 202 of device housing 205, as shown.

With further reference to FIG. 3, that portion of deployment tube lumen335 which extends along a length of distal-most portion 332 is sized tocontain distal end 322 of inner member 320 together with an entirety ofdevice 200. FIG. 3 shows fixation fingers 230 of the loaded device 200being held by distal-most portion 332 in the aforementioned extendedposition. With reference to FIG. 4A, a distal portion of tool 300, withan entirety of device 200 loaded in distal-most portion 332, may benavigated to a target implant site, for example, in the right atrium RA(or right ventricle RV), by advancing tool 300 through a venous systemof the patient, for example, from a femoral venous access site and upthrough the inferior vena cava IVC. A length of deployment tube 330,between handle 310 and a distal opening 303 of deployment tube 330, whentube 330 is in the position shown in FIG. 3, may be betweenapproximately 103 cm and approximately 107 cm, for example, to reach theright atrium RA from the femoral access site. According to someembodiments of the present invention, delivery tool 300 includesarticulating features to facilitate the navigation of the distal portionof delivery tool 300; for example, inner member 320 of delivery tool 300may include a pull wire (not shown) integrated therein and coupled toanother control member 311 of handle 310 that, when moved per arrow A,causes inner member 320 and deployment tube 330 to bend along distalportions thereof. Suitable construction detail for a delivery tool liketool 300 is described in co-pending and commonly assigned U.S. patentapplication Ser. No. 14/039,937, the description of which is herebyincorporated by reference.

According to some methods of the present invention, once an operator haslocated distal-most portion 332 in a chamber of the heart, for example,the right atrium RA, as shown in FIG. 4A, the operator can retractdeployment tube 330, per arrow W (FIG. 3), for example, by movingcontrol member 312 per arrow B (FIG. 3), to release fixation fingers 230to the relaxed position as shown in FIG. 4B. FIG. 4B illustrates eachfinger 230 having been exposed out through distal opening 303 ofdeployment tube 330 so that, in the relaxed position, each finger 230extends in a proximal direction and outward from device housing 205.Then, after releasing device fixation fingers 230, the operator mayadvance tool 300 and device 200 together to a target implant sitebetween folds of tissue, for example, pectinate muscle bands in rightatrial appendage 102, and, thus, wedge the exposed fixation fingers 230between opposing tissue surfaces as shown schematically in FIG. 4C. Withreference to FIG. 4C, distal end 322 of device inner member 320 may beemployed to provide a push force that assists in wedging fingers 230 sothat fingers 230 are in the aforementioned compressed state to holdelectrode 261 in intimate tissue contact. With reference back to FIG. 2,according to some embodiments, one or more of finger free ends 236includes a discrete radiopaque marker 238 attached thereto, for example,a platinum-iridium rivet like member. Optional marker(s) 238 may assistthe operator in assessing the fixation of device 200 at the implantsite. It should be noted that the compressed fingers 230, having asuper-elastic nature, hold device 200 in place at the implant site by aspring force (per the bold arrows of FIG. 4C), and that finger free ends236 are preferably configured to prevent penetration thereof withintissue at the implant site, while merely catching, or lodging againstopposing tissue surfaces. Furthermore, with reference to FIG. 4A, analternate implant site may be in the right ventricle RV, where fixationfingers 230 may be wedged between folds of tissue (trabeculae) in thearea of apex 103. It should be noted that the fixation fingers 230, asdescribed above in conjunction with FIGS. 2B-C, may also be formed froma polymer material, either individually or integrally with base ring239, wherein an appropriate polymer material and associated dimensionalspecifications essentially mimics that of fingers 230 formed from theaforementioned Nitinol, in terms of spring properties.

After wedging fingers 230 between opposing tissue surface, the operatormay evaluate pacing performance of electrode 261 before completelywithdrawing delivery tool 300 away from the implanted device 200. Thus,if the operator determines that the performance is not satisfactory, theoperator may advance distal-most portion 332 of deployment tube 330 backin a distal direction, for example, via control member 312 (FIG. 3),relative to device 200 and inner member 320 and over wedged fixationfingers 230 to move device 200 back into distal-most portion 332 withfingers 230 moved back into the extended condition, as shown in FIG. 3.Then the operator can move delivery tool 300 with the re-loaded device200 into proximity with an alternative implant site, retract deploymenttube 330 again to expose and release fingers 230 into the relaxedcondition (FIG. 4B), and then advance tool 300 toward the other site towedge the exposed fingers 230 between opposing tissue surfaces at theother site (FIG. 4C).

FIG. 5A is a perspective view of an implantable medical device 500,according to some additional embodiments; and FIG. 5B is a cross-sectionview through a portion of device 500, according to an exemplaryconstruction of some embodiments. FIG. 5A illustrates device 500 beingsimilar to device 200 but including a pacing extension 560 on which apacing electrode 561 is mounted, in lieu of electrode 261 of device 200.FIG. 5A further illustrates extension 560 including a preformedcurvature located in proximity to, and proximal to electrode 561. Adiameter of extension 560 may be approximately 0.05 inch (1.3 mm); anoverall length of extension 560 may be approximately 0.6 inch (15 mm);and the curvature, preferably in a single plane, is defined by a radiusR, which may be approximately 0.2 inch, according to an exemplaryembodiment. According to the illustrated embodiment, electrode 561 islocated in close proximity to a distal tip 565 of extension 560, whichtip 565 is preferably tapered. FIG. 5B illustrates distal tip 565 beingslightly enlarged from a remainder of extension 560; and, according tosome embodiments, tip 565 includes electrode 561, which forms at least aportion of the taper, and a relatively soft medical grade siliconerubber member 567, which may include a steroid embedded therein. Theillustrated contour of electrode 561 may help electrode 561 to makebetter tissue contact when tip 565 lies adjacent to tissue, for example,as illustrated in FIG. 6C. According to an exemplary embodiment, adiameter of tip 565 (as shown in FIG. 5B) is approximately 0.07 inch(1.8 mm), and a surface area of electrode 561 is approximately 5.8 mm².Electrode 561 may be formed from a platinum iridium alloy.

FIG. 5B further illustrates pacing extension 560 being formed by acoiled multi-filar conductor 562 (e.g., MP35N alloy) enclosed within ajacket of insulation 564 (e.g., medical grade polyurethane), and anexemplary junction between electrode 561 and conductor 562, which may besecured by crimping and/or welding according to methods known in the artof implantable medical electrical leads. According to the illustratedembodiment, conductor 562 electrically connects electrode 561 to theaforementioned pulse generator contained within device housing 205, forexample, via a feedthrough assembly constructed according to methodsknown in the art of implantable medical devices.

Turning now to FIG. 6A, as was described above for device 200, device500 is loaded into distal-most portion 332 of delivery tool 300 (FIG.3), such that fixation fingers 230 are in the extended condition. FIG.6A illustrates fingers 230 extending in a distal direction and alongsidepacing extension 560 within distal-most portion 332. After device 500 isloaded, the operator may navigate delivery tool 300, with device 500completely contained therein, through the patient's venous system, forexample, from a femoral venous access site, up through the inferior venacava IVC, and into a chamber of the heart, for example, the right atriumRA, as shown in FIG. 4A. In some preferred embodiments, pacing extension560 extends distally beyond extended fingers 230 so that the operatormay withdraw deployment tube 330, per arrow W, just enough to exposeelectrode 561 out through distal opening 303 thereof, as shown in FIG.6B. According to some exemplary embodiments, extension 560 can extendapproximately 2 to 4 mm beyond opening 303 without fingers 230 beingexposed, so that the operator can advance tool 300 to one or morepotential implant sites, where electrode 561 makes contact, to mapelectrical activity and/or to check pacing thresholds. According to somemethods, after finding a desired implant site in this manner, theoperator can pull back tool 300 and device 500 together, for example, tothe position shown in FIG. 6B, and then retract deployment tube 330 evenfurther, with respect to device 500 and inner member 320, to exposefixation fingers 230 out from distal opening 303, thereby releasingfingers 230 to the relaxed condition (FIG. 4B). Then, as describedabove, the operator can advance tool 300 and device 500 together back tothe desired implant site, for example, between pectinate muscle bands inright atrial appendage 102, and, thus, wedge the exposed fixationfingers 230 between opposing tissue surfaces as shown schematically inFIG. 6C, to hold device 500 at the implant site with electrode 561making intimate tissue contact. With further reference to FIG. 6C, itmay be appreciated that the length of pacing extension 560 serves toseparate that portion of the implant site at which electrode 561 makescontact with that portion of the site at which fixation fingers 230 makespring contact (e.g., per bold arrows), so that any inflammationassociated with the fixation fingers contact may not impair chronicpacing thresholds.

FIG. 7 is a plan view of an implantable medical device 700, according tosome alternate embodiments. FIG. 7 illustrates device 700 being similarto device 200 of FIG. 2, in that device 700 includes hermetically sealedhousing 205, which may be overlaid with an insulative layer, forexample, medical grade polyurethane, parylene, or silicone, and whereinelectrode 262 may be formed by removing a portion of the insulativelayer to expose the metallic surface of housing 205. FIG. 7 furtherillustrates device 700 including a fixation member, which is formed by aplurality of fixation fingers 730, for example, eight fingers, spacedapart from one another around a perimeter of a distal end 201 of housing205, and an electrode 761, which is spaced distally apart from distalend 201 of housing 205 by a distance y, which may be approximately 2 mm,for example, to allow electrode 761 to make better tissue contact alonguneven surfaces such as between pockets created by relatively smallpectinate muscle bands in right atrial appendage 102 (FIG. 1). Withreference back to FIG. 2A, it should be noted that electrode 261 ofdevice 200 may be similarly spaced from distal end 201 of housing 205.Electrode 761, like electrode 261, may be coupled to the controller ofdevice 200 by a hermetic feedthrough assembly to function in conjunctionwith electrode 262 for bipolar pacing and sensing.

According to the illustrated embodiment, fixation fingers 730 are formedfrom a flexible polymer material, for example, medical grade siliconerubber or polyurethane, and, in a relaxed condition, extend proximallyfrom distal end 201 of device housing 205, and outward therefrom at anangle α relative to a longitudinal axis of device housing 205. Angle αmay be approximately 60 degrees, and a length of each finger 730, from afixed end 735 to a free end 736 thereof, may be approximately 0.2 inch(5 mm). Like the above described fixation fingers 230, fingers 730 areconfigured to hold electrode 761 in intimate tissue contact at a targetimplant site, for example, within right atrial appendage 102 or withinright ventricle RV in proximity to apex 103 (FIG. 1), but, beingrelatively softer than Nitinol, silicone rubber or polyurethane fingersmay be less likely to migrate through tissue at the implant site overtime.

In the foregoing detailed description, the invention has been describedwith reference to specific embodiments. However, it may be appreciatedthat various modifications and changes can be made without departingfrom the scope of the invention as set forth in the appended claims.

We claim:
 1. A method for deploying an implantable medical device to atarget implant site located in a chamber of a patient's heart, thedevice comprising a hermetically sealed housing containing an electroniccontroller, a pacing electrode electrically coupled to the controllerand mounted in proximity to a distal end of the housing, and a pluralityof elastically deformable fixation fingers spaced apart from one anotheraround a perimeter of the distal end of the housing an elasticallydeformable between an extended condition, a relaxed condition, and acompressed condition, wherein each fixation finger includes a firstsegment and a second segment, each first segment extending from a fixedend of the corresponding finger to the corresponding second segment, andeach second segment extending from the corresponding first segment to afree end of the corresponding finger, and wherein, when the plurality offingers are in the relaxed condition, each free end is closer to thehousing than when the plurality of fingers are in the extended conditionand, when the plurality of fingers are in the compressed condition, eachfree end is closer to the housing than when the plurality of fingers arein the relaxed condition, the method comprising: loading an entirety ofthe medical device into a distal-most portion of a deployment tube of adelivery tool such that a proximal end of the device abuts a distal endof an inner member of the tool, and such that each fixation finger isdeformed into the extended position with each finger extending distallyfrom the distal end of the housing, the distal-most portion of the toolholding the fixation fingers in the extended position, and the innermember of the tool extending within a lumen of the deployment tube;navigating the delivery tool with the device loaded therein through avenous system of the patient to locate the distal-most portion of thetool in the chamber of the heart; retracting the deployment tube of thedelivery tool with respect to the inner member and the loaded device toexpose the fixation fingers out through a distal opening of the lumen ofthe deployment tube, the distal opening terminating the distal-mostportion of the tube, and the exposed fixation fingers being released tothe relaxed condition with each finger extending in a proximal directionand outward from the housing; and advancing the delivery tool toward thetarget implant site, after retracting the deployment tube, and therebywedging the exposed fixation fingers into the compressed condition andbetween opposing tissue surfaces of the implant site, the wedged fingersbeing compressed to hold the pacing electrode of the device in intimatetissue contact by a spring force that passively fixates the implantablemedical device to tissue at the implant site without penetrating thetissue at the implant site.
 2. The method of claim 1, furthercomprising: advancing the deployment tube of the delivery tool, withrespect to the inner member and the device, over the wedged fixationfingers to move the fingers back into the extended condition within thedistal-most portion; pulling the delivery tool back away from the site,with the fixation fingers in the extended condition; moving the deliverytool into proximity with another target implant site within the chamberof the heart; retracting the deployment tube of the delivery tool withrespect to the inner member and the device to expose the fixationfingers out through the distal opening of the lumen of the deploymenttube so that the fixation fingers are again released to the relaxedcondition; and advancing the delivery tool toward the other targetimplant site, after retracting the deployment tube, and thereby wedgingthe exposed fixation fingers between opposing tissue surfaces of theother implant site, the wedged fingers being compressed to hold thepacing electrode of the device in intimate tissue contact.
 3. The methodof claim 1, wherein the pacing electrode of the device is located distalto the free end of each fixation finger, when the proximal segment ofeach is in the extended condition.
 4. The method of claim 1, wherein,when the first segment of each fixation finger is in the relaxedcondition, each first segment extends in an arc, distally and outwardlyfrom the corresponding fixed end, the arc enclosing an angle of at leastninety degrees.
 5. The method of claim 1, wherein each device fixationfinger further includes a discrete radiopaque marker located inproximity to the free end thereof.
 6. The method of claim 1, wherein thedevice further includes a pacing extension on which the pacing electrodeis mounted, the pacing extension extending distally from the distal endof the housing and electrically coupling the pacing electrode to theelectronic controller, and the method further comprises: retracting thedeployment tube of the delivery tool with respect to the inner member ofthe tool and the loaded device to expose the pacing electrode withoutexposing the fixation fingers, prior to retracting the deployment tubeto expose the fixation fingers; advancing the delivery tool toward thetarget implant site to bring the exposed pacing electrode into contactwith tissue at the site, prior to advancing the tool with the fixationfingers exposed; evaluating pacing performance of the pacing electrodewhen in contact with the tissue, prior to retracting the deployment tubeto expose the fixation fingers; and pulling the delivery tool back awayfrom the site, after evaluating the pacing performance, and prior toretracting the deployment tube to expose the fixation fingers.
 7. Themethod of claim 6, wherein the second segment of each fixation fingerextends in a distal direction, alongside the pacing extension, when thefixation finger is in the extended condition.
 8. The method of claim 6,wherein: the pacing electrode of the device is located in closeproximity to a distal tip of the pacing extension of the device; and thepacing extension of the device includes a pre-formed curvature inproximity to and proximal to the pacing electrode.
 9. The method ofclaim 6, wherein the pacing extension of the device includes a tapereddistal tip, and the electrode of the device forms at least a portion ofthe tapered distal tip.